Claar Pronk was given her first camera, a Kodak box, at the age of fourteen. Her father was Town Clerk in De Bilt near Utrecht, and a policeman friend allowed her to take pictures of people under arrest. She developed her photographs in her own darkroom at home. In 1935, after she had left school, a careers advice test came up with four options: social worker, librarian, analyst or photographer. Pronk duly left for Paris, where she trained at the Publiphot international photography school directed by Mme Gertrude Fuld, who had done theatre photography in Munich. Pronk learned colour photography from Paul Kowaliski. From September 1935 to Christmas 1936 she lived in Montmartre. In 1937 she went into business as a photographer in Utrecht and subsequently set up a colour studio for Elsevier. Pronk was one of the few Dutch photographers who worked in colour. She obtained her first major assignment, for the 1937 World Exhibition at Paris, through architect S. van Ravesteyn, who was involved in the Exhibition.
Until 1943 she worked on assignment for the Bosschaert advertising agency in Amsterdam, the Centraal Museum in Utrecht and Elseviers printers. Her photographs were published in calendars and books and exhibited at the Utrecht artists' society Kunstliefde and at various mixed shows. In 1943 she married the dentist H.H.W. Verdenius and moved with him to Olst. She closed her Utrecht studio and more or less gave up photography. Her husband died suddenly in 1961, by which time she had six children; the youngest daughter was seven, the oldest fifteen. In 1964 Claar Pronk resumed her career under the name Claar Verdenius. Her first photographs were chiefly of children. Later, she received commissions from the sculptors Wout Maters and Hak Hekman, the Wouda architects office and in 1975-1976 the Academie voor Expressie door middel van Woord en Gebaar (Academy of verbal and gestural expression). Other commissions came through the architects van Ravesteyn and Rietveld, and she also did fashion photography for the women's magazine Libelle.
Before the war Claar Pronk had been a member of the Vereniging voor Ambachts- en Nijverheids Kunst (Arts and crafts society). After the war she remained briefly in contact with prominent GKf members, but never joined. She tended to undercut the prices of her colleagues, who were understandably not very pleased. In 1950 she collaborated with writer Jaap Romijn on Het Hart van Nederland, a book about the city and province of Utrecht; in 1958 she published Kinderen kennen geen grenzen (Children are unaware of frontiers). In 1972 she discovered the School of Philosophy and began to frequent Amsterdam, where she photographed the new Werktheater and actress Loes Luca. She stopped taking photographs in 1980.